Although the invention in U.S. Pat. No. 4,588,493 is effective to accomplish the intended result, some cracks in the electrolyte tube have occurred due to thermal shock resulting from non-uniform temperature gradients. For example, when the furnace temperature is lowered, the metal disk 82 in FIG. 1 of that patent quickly cools the tip or ball nose 7 of the electrolyte tube 28 in FIG. 2. The fast cooling occurs because the disk 82 which is metal, has a high thermal conductivity and consequently heat is rapidly removed from the tip of the ceramic electrolyte tube 28 which is in line contact with disk 82. In addition, when the furnace temperature is increased, the metal disk 82 initially heats up more rapidly than the ceramic electrolyte tube. In this case, heat flows from the metal disk to the surface of the ceramic electrolyte tube which is in line contact with the metal disk 82. Both of the above cases of cooling and heating of the furnaces produces a non-uniform temperature distribution at the tip of the electrolyte tube. This non-uniform temperature distribution produces a non-uniform thermal stress distribution at the tip of the electrolyte tube, (i.e., when the furnace temperature changes a temperature difference develops between the outside surface and the interior of the electrolyte tube). According to Richerson, Modern Ceramic Engineering published by Marcel Dekker, Inc., 1982, page 139, "Thermal shock refers to the thermal stresses that occur in a component as a result of exposure to a temperature difference between the surface and interior, or between various regions of the component." These thermal stresses can produce micro cracks in the tip of the electrolyte tube. It should also be noted, (e.g. see U.S. Pat. No. 4,101,404, Column 1, Lines 25-27), that the electrolyte tube which is a ceramic material, is hard and brittle, and is thus very susceptible to thermal shock. For example, if the oxygen probe is installed in a hot furnace, it must be slowly pushed into the furnace approximately at the rate of 1 inch/min. to avoid thermal shock. Also, if the oxygen probe is removed from a hot furnace, it is also important to remove it slowly (Approx. 1 inch/min.) to avoid thermal shock. Non-uniform temperature distributions that may produce cracks in the electrolyte tube from thermal shock can also be caused by thermal cycling. Thermal cycling results from the fluctuation in temperature of the radiant tube or other heat sources which supplies the heat to the furnace. The thermal cycling in conjunction with openings in the metal alloy sheath 17 in FIG. 1 of U.S. Pat. No. 4,588,493, will also produce a non-uniform temperature distribution on the electrolyte tube 28. Loading and/or unloading of work into the furnace will also cause non-uniform temperature distribution. If the electrolyte tube, which separates the reference air in the interior of the electrolyte tube from the furnace atmosphere, develops a crack, gas may leak through the crack and alter the gas composition locally at the electrode region. Any change in the gas composition that produces a change in the oxygen partial pressure will affect the voltage developed by the oxygen sensor and cause an error in its accuracy.